The invention relates to parallel plate electron multipliers. In particular, the invention relates to such devices employing electrostatic fields for containing the electron cloud and for reducing ion feedback.
A continuous dynode parallel plate electron multiplier (PPM) 10 illustrated in FIGS. 14 and 15 creates a detectable electron avalanche 12 when stimulated by a photon or an energetic charged particle 14. In the device shown, a pair of parallel plates 16-18 carry dynodes 20-22 formed thereon of a suitable material with an appropriate resistance and secondary electron yield. The dynode material is uniformly distributed on the confronting parallel surfaces of the plates 16, 18 so that the active portions of the dynodes 20-22 face each other.
The plates 16-18 are separated by a gap (G) 28 and the device 10 has a length (L) 30 from its input end 32 to its output end 34. The ratio of L over G is about 20:1 or better for satisfactory electron multiplication output.
Electrical connections 36-38 are made from a high voltage supply (40) between the input end 32 and the output end 34 of the dynodes 20 and 22 as shown. The high voltage supply 40 biases the front of the device 10 negatively with resistance in the semiconducting range experience electrical conduction down the length of the device thereby creating a uniform gradient in potential down the center axis 42 of the PPM. In the simplified illustration of FIG. 15, a sufficiently energetic photon or charged particle 14 impinging on the dynode 22 at input end 32 of the PPM 10 causes secondary electrons 44 to be emitted from the dynode 22 at the point of the impact. These secondary electrons 44 are typically emitted with some energy in the direction normal to the surface of the dynode 22. The initial energy causes secondary electrons 44 to travel across the gap 28 between the plates 16-18. Simultaneously, the electrons are accelerated down the length of the device 10 under the influence of the electric field produced by the high bias voltage 40. The electrons continue to accelerate until they strike the opposite dynode 20. Bias voltages, plate spacing and emissive dynode layers are chosen so that the electrons gain sufficient impact energy to create an average number of secondary electrons greater than 1. Each new electron is accelerated away from its origin until it strikes an opposing dynode. This process repeats itself as the electrons progress down the length of the device. The number of electrons in the cascade increases geometrically with each strike resulting in an electron avalanche 12 at the output end 34.
Although parallel plate electron multipliers have a relatively simple configuration and may be processed using less complicated techniques, PPMs have a number of problems which discouraged their implementation. Of particular concern are the containment of the electron avalanche between dynode surfaces and ion feedback. With respect to containment, as the electron density increases, the repulsive force between the secondary electrons tends to direct them out the open sides 46 of the dynode region (FIG. 14). This limits the size of the charge cloud and the gain of the multiplier. With respect to ion feedback, the increasing avalanche of secondary electrons 44 near the output end 34 of the device enhances the probability of ionizing residual gas or stimulating desorption of ionized species 48 from the dynode surfaces 20 and 22 (FIG. 15). These ions are accelerated towards the input end 32 where they can strike the dynode surfaces and generate a new electron avalanche. This phenomenon is referred to as ion feedback and has a deleterious effect on the signal-to-noise ration of the device.
In channel electron multipliers, that is devices formed in tubular or capillary configuration, these problems are corrected by the geometry of the device, where the capillary channel serves to contain the electron cloud. Further, curvature of the channel forces ions to collide with the channel wall close to the output end of the device thereby reducing the size of the resulting ion feedback pulses. However, CEMs often require more complex processing and are often too large for a particular application.